Volume 127, Issue 1, Pages 166-178 (July 2004) Splanchnic and pelvic mechanosensory afferents signal different qualities of colonic stimuli in mice  Stuart M. Brierley, R.Carter W. Jones, Gerald F. Gebhart, L.Ashley Blackshaw  Gastroenterology  Volume 127, Issue 1, Pages 166-178 (July 2004) DOI: 10.1053/j.gastro.2004.04.008

Figure 1 (A) Four types of lumbar splanchnic afferent fiber classified on the basis of their receptive field location and response to mechanical stimuli. (i) Mesenteric afferents responded in a graded manner to an ascending series of probing stimuli (70 mg–4 g). (ii) Serosal afferents were activated only by probing their receptive field. (iii) Muscular afferents were activated by probing and maintained circular stretch but did not respond to fine mucosal stroking (10 mg). (iv) Mucosal afferents were activated by probing and fine mucosal stroking but did not respond to circular stretch. (B) Four types of pelvic afferent classified on the basis of their responses to mechanical stimuli. (i) Muscular/mucosal afferents were activated by probing, stretch, and fine mucosal stroking (10 mg). (ii) Serosal afferents were activated by probing of their receptive field and did not respond to maintained circular stretch or fine mucosal stroking. (iii) Muscular afferents were activated by probing and circular stretch but did not respond to fine mucosal stroking. (iv) Mucosal afferents were activated by probing and fine mucosal stroking but not stretch. Upper traces show instantaneous firing frequency and lower traces show raw electrophysiologic data. Horizontal bars indicate application of stimulus. Scale bars apply throughout except where indicated. Gastroenterology 2004 127, 166-178DOI: (10.1053/j.gastro.2004.04.008)

Figure 2 Distribution and proportions of afferent classes recorded from the splanchnic and pelvic pathways. (A) Splanchnic receptive fields were concentrated on or near the mesenteric attachment and were scattered down the entire length of the colon except for the rectum and anal canal. (B) The majority of splanchnic afferents encountered were mesenteric (striped segment) and serosal afferents (grey segment), with the remaining few composed of muscular (black segment) and mucosal afferents (white segment). (C) Pelvic afferent receptive fields were scattered across the entire width of the colon and were generally clustered in the lower region of distal colon and rectum. No receptive fields were found on the mesentery. (D) The largest population of pelvic afferents encountered were serosal (grey segment), with similar proportions of muscular (black segment), mucosal (white segment), and muscular/mucosal (checkered segment) afferents. IMG, inferior mesenteric ganglion; LSN, lumbar splanchnic nerve; PN, pelvic nerve; MPG, major pelvic ganglion. Gastroenterology 2004 127, 166-178DOI: (10.1053/j.gastro.2004.04.008)

Figure 3 Mechanosensitivity, adaptation, and activation characteristics of splanchnic and pelvic afferents to graded stimulation with von Frey hairs. (A and D) Stimulus-response functions of splanchnic and pelvic afferents to probing. All afferent subtypes from both pathways displayed graded responses to increasing probing stimuli (70 mg–4 g). Grouped responses to probing of splanchnic and pelvic afferents were significantly different (P < 0.05). However, within each pathway, probing responses of individual afferent classes did not differ significantly nor were the slopes of stimulus-response functions significantly different (P > 0.05 for all analyses). (B and E) Adaptation profiles of splanchnic and pelvic afferent responses during a 3-second application of a 1-g probe. The adaptation curves of all splanchnic afferents displayed similar slopes (B; P > 0.05). Among pelvic afferents, the adaptation profile of mucosal afferents was significantly shallower than those of serosal or muscular afferents (E; P < 0.05). (C and F) von Frey hair force required to activate splanchnic and pelvic afferents. Significantly fewer serosal and muscular afferents in the LSN were activated by probe intensities ≤1g, compared with similar afferents in the PN (P < 0.05). (C) Mucosal afferents were significantly more sensitive to probing than all other LSN afferent classes, with 100% recruited by the lowest probe intensity tested (P < 0.05). A probing force of 2 g was required to activate all LSN afferents. (F) In contrast to splanchnic afferents, muscular/mucosal afferents were the most sensitive to probing among all pelvic afferents, with a significantly different percentage responding curve compared with the other pelvic afferent classes. In addition, more mucosal afferents were activated by probing intensities ≤1 g than were serosal afferents (P < 0.05). A probing force of 1 g was required to activate 100% of PN afferents. Gastroenterology 2004 127, 166-178DOI: (10.1053/j.gastro.2004.04.008)

Figure 4 Comparison of serosal and muscular afferents between splanchnic and pelvic pathways. (A) Stimulus-response functions of splanchnic (n = 19) and pelvic (n = 18) serosal afferents to probing. Serosal afferents from both pathways displayed graded responses to increasing probing stimuli (70 mg–4 g). However, pelvic serosal afferents were significantly more sensitive to probing, displaying larger stimulus-response functions (P < 0.001; 2-way ANOVA) and steeper slopes than splanchnic serosal afferents (P < 0.001; PN slope, 29.63 ± 4.31 vs. LSN slope, 15.02 ± 1.88). (B) Adaptation profiles of serosal afferents recorded from the PN (n = 18) and LSN (n = 19) to a 3-second, 1-g probing stimulus. The pelvic response was significantly larger throughout (P < 0.001, n = 18 vs. n = 19, respectively) and fitted a linear regression, whereas splanchnic data was nonlinear. However, splanchnic serosal afferents demonstrated significantly faster adaptation over the first 0.5 seconds of the response (P < 0.01) and more complete adaptation than pelvic serosal afferents. (C) Stimulus-response functions of muscular afferents in the LSN (n = 5) and PN (n = 12) to graded probing stimuli. Muscular afferents from both pathways displayed graded responses to increasing probing stimuli (70 mg–4 g). However, pelvic muscular afferents are more sensitive to probing, displaying significantly larger stimulus-response functions (P < 0.001) with steeper slopes than splanchnic muscular afferents (P < 0.001; PN slope, 30.89 ± 5.5 vs. LSN slope, 9.01 ± 3.3). (D) Adaptation profiles of splanchnic (n = 5) and pelvic (n = 12) muscular afferents to a 3-second, 1-g probing stimulus. Pelvic muscular afferents responded significantly higher throughout the stimulus than splanchnic muscular afferents (P < 0.001); however, there was no difference in the slopes of the 2 curves (P > 0.05). (E) Circular stretch stimulus-response functions of splanchnic (n = 5) and pelvic (n = 12) muscular afferents. All pelvic and splanchnic muscular afferents responded to the full range of stretch stimuli (1–5 g). However, pelvic muscular afferents were significantly more responsive to stretch, displaying higher stimulus-response functions (P < 0.001) with steeper slopes than those of splanchnic muscular afferents (P < 0.001; PN slope, 0.30 ± 0.03 vs. LSN slope, 0.11 ± 0.02). (F) Adaptation profiles of splanchnic and pelvic muscular afferents to a 1-minute, 3-g stretch. Pelvic muscular afferents displayed significantly more spikes per 10-second bin across the entire stimulus period than splanchnic afferents (P < 0.001). Both splanchnic and pelvic muscular afferents adapted over the first 20 seconds of the stimulus. Although pelvic afferents continued to discharge over the remainder of the stimulus, splanchnic afferents returned to their spontaneous level of firing 20 seconds after the stimulus onset. Thus, muscular afferents in the LSN adapted more completely to circular stretch than those in the PN. Note that pelvic afferents showed no spontaneous activity. Gastroenterology 2004 127, 166-178DOI: (10.1053/j.gastro.2004.04.008)

Figure 5 Comparison of dynamic properties of pelvic afferent classes. (A) Mucosal stroking stimulus-response function of PN afferents. All 4 classes demonstrated graded responses to mucosal stroking (10–1000 mg). Notably, mucosal (n = 13) and muscular/mucosal (n = 13) afferents were the only classes to respond to the 10-mg filament and exhibited stimulus-response functions to mucosal stroking that were significantly higher than those of pelvic serosal (n = 18) and muscular (n = 12) afferents (P < 0.001). (B) Circular stretch stimulus-response functions of muscular and muscular/mucosal pelvic afferents. All muscular and muscular/mucosal afferents responded to the full range of stretch stimuli (1–5 g). Two subclasses of muscular/mucosal afferents could be distinguished, based on their responses to stretch: a high-responding population, with responses significantly different than muscular afferents (P < 0.001, n = 6) and a low-responding population, with responses similar to those of muscular afferents (P > 0.05, n = 7). (C) Adaptation profiles of muscular and muscular/mucosal pelvic afferents to a 1-minute, 3-g stretch. High-responder muscular/mucosal afferents displayed significantly more spikes per 10-second bin across the entire 1-minute stimulus period compared with both low-responder muscular/mucosal and muscular afferents (P < 0.001). No significant difference in the rate of adaptation, defined as the slope of the adaptation curve, was observed among pelvic muscular and muscular/mucosal afferents (P > 0.05). Gastroenterology 2004 127, 166-178DOI: (10.1053/j.gastro.2004.04.008)